The present invention relates generally to tracked mobile machines, and more specifically to a star carrier roller assembly for a tracked mobile machine and a method for assembling the star carrier roller assembly.
Several tracked mobile machines include a rotating endless track chain. An upper portion of the track chain is guided and supported by carrier rollers and a bottom portion is guided by bottom rollers. Often, the bottom rollers and the carrier rollers both include a cylindrical body that rotates as the track chain crosses over them. Some of these tracked mobile machines, such as those used in the timber industry, must be able to operate during relatively cold weather and/or when mud or other undesirable substances have attached to the carrier rollers. During cold weather or when mud or other undesirable substance attach to the carrier rollers, there is insufficient friction between the carrier rollers and the rotating track chain to also rotate the carrier rollers. Thus, the track chain will slide over the stationary carrier rollers, resulting in wear on the carrier roller. The wear causes the carrier roller to lose its circular shape, making it even more difficult for the passing track chain to rotate the carrier roller. The end result is a necessity to prematurely replace the worn carrier roller.
Over the years, engineers have developed engagement apparatuses that can ensure rotation of carrier rollers attached to these tracked mobile machines operating in relatively harsh conditions, such as tracked mobile machines used in the timber industry. For instance, carrier rollers, such as those shown in U.S. Pat. No. 6,203,124 B1, issued to Meyer et al., on Mar. 20, 2001, include driving pins, arranged equidistantly on a coaxial circumferential circle. A driving lug of the track chain engages between two of the driving pins to ensure that the roller carrier will rotate with the rotating track chain. Although the Meyer carrier roller does engage the track chain such that it ensures rotation of the carrier roller, assembly of the Meyer carrier roller may be time consuming and costly. Further, the Meyer carrier roller does not engage all types of track chains, such as track chains that do not include protruding driving lugs. Thus, use of the Meyers strategy would necessitate costly modifications to existing track chains.
Another approach known in the art to ensuring rotation of the carrier roller is to attach a sprocket to the existing carrier rollers. A sprocket attached to a carrier roller is referred to as a star carrier roller assembly. The sprockets generally include five teeth that engage the track chain as it rotates. When the teeth engage the track chain, the carrier roller attached to the sprocket must also rotate. Although the sprocket, without the carrier roller, would engage the track chain, the interactions of the track chain and the sprocket, alone, prevents smooth operation of the tracked mobile machine. Although these star carrier roller assemblies are shaped to ensure rotation of the carrier rollers, there is room for improvement in the design of the star carrier roller assemblies.
A typical track chain includes an endless chain of connected link assemblies. Generally, each link assembly includes a right link and left link and a bushing perpendicularly attached to both links. The links travel across a wear surface of the carrier roller, and the teeth engage the bushing. Each time the tooth of the sprocket comes in contact with the bushing, the bushing may rub against the tooth, causing sprocket and/or bushing wear. In addition, because the roller has a smaller radius than the sprocket, when the tooth of the sprocket comes in contact with the bushing, the roller will be rotating slower than the track chain. Thus, the contact between the tooth and the track chain will cause the track chain to momentarily jerk. The wear surface of the roller that is in contact with the track chain when the jerking movement occurs may develop a flat spot due to this wearing.
Those skilled in the art will appreciate, that although there may always be some wear occurring between the track chain and the star carrier roller assembly, the geometry of the sprocket can exacerbate the wear on the roller, the bushing, and the sprocket. For instance, one manufacturer has developed a star carrier roller assembly that includes relatively short and wide teeth, which can cause unacceptable sprocket and bushing wear. In addition, this known design includes a relatively steep slope on the teeth, resulting in the bushing sliding down the tooth, which further increases bushing and sprocket wear. Thus, although this design may decrease the wear on the roller carrier by attaching the sprocket, it may lead to premature wear of the sprocket and the bushing.
Contrary to the previous approach, another manufacturer has designed a star carrier roller assembly that includes relatively lengthy, narrow teeth that engage the track chain. There is a larger distance between the sprocket teeth than there is with the previously discussed design, resulting in less contact between the bushing and the sprocket. However, the teeth of the new design have a negative slope, meaning that the teeth get slightly wider as they extend from the carrier roller. In addition, this star carrier roller assembly has a relatively long tooth tip, meaning that the tip of the tooth is significantly rounded. Although this design may reduce the wear on the sprocket and the bushing of the track chain, the negative slope and the relatively rounded tooth tip of the star carrier roller can result in premature carrier roller wear.
The life of a carrier roller that suffers from excessive wear may be shortened by forty to sixty percent. Further, wear can cause the respective component to function improperly. In addition to the wear, the attachment of the sprockets to the carrier roller causes the tracked mobile machine to operate less smoothly. Thus, some star carrier roller assemblies limit bushing and sprocket wear, but result in increased carrier roller wear and excessive vibrations. On the other hand, other star carrier roller assemblies result in relatively smooth operation and less carrier roller wear, but increase bushing and sprocket wear.
Moreover, there is room for improvement in the method of assembling star carrier roller assemblies. Often when star carrier roller assemblies are manufactured, the sprocket is attached to the previously assembled carrier roller. During assembly of the carrier roller, bearings are mounted inside a cylindrically shaped body comprised of tempered steel, and a rubber seal is attached to prevent mud and other undesirable substances from entering the body and interfering with the bearings. When the sprocket is welded to the hardened steel body, the heat created by the welding can temper the steel body, thereby reducing the roller""s hardness and subjecting it to hastened wear. Further, during the welding of the sprocket, the heat can affect the rubber comprising the seals, and thereby damaging the seals. Although prior to welding the sprocket onto the carrier rollers body, the seals could be removed from the body, this process increases the costs of manufacturing the star carrier roller assemblies.
The present invention is directed to overcoming one or more of the problems set forth above.
In one aspect of the present invention, a tracked mobile machine includes a track chain assembly attached to a machine body and including a pitch length and a bushing diameter. At least one star carrier roller assembly is attached to the machine body and includes a sprocket positioned adjacent to a carrier roller. The carrier roller has a cylindrical wear surface. The sprocket has a plurality of teeth, and each tooth has a tooth width and a tooth angle. The tooth angle is greater than or equal to zero degrees (0xc2x0). A ratio between the tooth width and a difference of the pitch length and the bushing diameter is less than 0.3.
In another aspect of the present invention, a star carrier roller assembly includes a carrier roller with a cylindrical wear surface and a sprocket that is positioned adjacent to the carrier roller. The sprocket has a plurality of teeth, and each tooth includes a tooth angle that is equal to or greater than zero degrees (0xc2x0). The sprocket has a plurality of non-contact zones, each of which define a non-contact zone angle. The non-contact zones are separated from a plurality of contact zones, each of which define a contact zone angle. The contact zone angle is less than the non-contact zone angle.
In yet another aspect of the present invention, a star carrier roller assembly is assembled by maintaining a wear surface on a carrier roller less than three hundred degrees Fahrenheit (300xc2x0 F.) while attaching a sprocket to the carrier roller.